Heterojunction bipolar transistor with a thin silicon emitter

ABSTRACT

A heterojunction bipolar transistor having a thin, lightly doped, silicon emitter disposed on a silicon-germanium base layer exhibits low emitter resistance and low emitter-base capacitance. The lightly doped silicon emitter maintains the bandgap differential between silicon-germanium and silicon. The silicon emitter is fabricated such that the silicon emitter will be substantially depleted at zero bias, resulting in low emitter-base resistance and emitter resistance.

BACKGROUND OF THE INVENTION

This invention relates, in general, to semiconductor devices, and moreparticularly, to a heterojunction bipolar transistor.

Heterojunction bipolar transistors exhibit electrical characteristicswhich are advantageous over the electrical characteristics ofhomojunction bipolar transistors. Silicon-germanium heterojunctionbipolar transistor processing is compatible with existing siliconprocessing. Thus, silicon-germanium heterojunction bipolar transistorsare preferred over other heterojunction bipolar transistors. Inparticular, silicon-germanium heterojunction bipolar transistors exhibithigh emitter injection efficiency, reduced charge storage in theemitter, reduced or eliminated hole injection into the emitter. Thesecharacteristics are obtained because of the bandgap differential betweenthe silicon and silicon-germanium metallurgical junction.

A silicon-germanium heterojunction bipolar transistor of the prior artconsists of an N-type silicon collector, a P-type silicon-germaniumbase, and an N-type polysilicon layer. The N-type dopant from thepolysilicon is diffused into the base to form an emitter region. Theproblem with this structure is that boron diffuses into the polysiliconlayer or arsenic diffuses into the silicon-germanium base during theformation of the emitter. This diffusion degrades the bandgapdifferential by moving the metallurgical junction from thesilicon-germanium and polysilicon interface into either the polysiliconlayer or into the silicon-germanium base. Thus, the advantageouselectrical characteristics described above are not exhibited.

A way of maintaining the metallurgical junction at the silicon-germaniumand polysilicon interface is to prevent the diffusion of boron orarsenic from the respective layers. A structure in which the polysiliconlayer is used as the emitter would solve this problem because thediffusion of boron or arsenic can be prevented. However, in thisstructure, the interface between the polysilicon layer and thesilicon-germanium layer is poor, which results in the transistorexhibiting poor electrical characteristics, such as high leakage.

A transistor which solves this interface problem has been disclosed byKing et al, in an article entitled, "Si/Si_(1-x) Ge_(x) HeterojunctionBipolar Transistors Produced by Limited Reaction Processing," publishedin IEEE Electron Device Letters, Vol. 10, No. 2, on Feb. 1989. The useof a thick silicon emitter of approximately 4,000 angstroms inthickness, instead of the polysilicon emitter eliminates the interfaceproblem, and maintains the bandgap differential between thesilicon-germanium layer and the silicon emitter layer in this case.However, in this structure, the thick silicon emitter must be lightlydoped to avoid breakdown voltage problems and to avoid high emitter-basecapacitance. A lightly doped, thick emitter exhibits high emitterresistance. Thus, it would be desirable to fabricate a heterojunctionbipolar transistor in which the bandgap differential is ensured and alsowhere emitter resistance and capacitance is low.

Accordingly, it is an object of the present invention is to provide animproved heterojunction bipolar transistor.

Another object of the present invention is to provide a heterojunctionbipolar transistor having low emitter-base capacitance and low emitterresistance.

A further object of the present invention to provide a heterojunctionbipolar transistor in which the bandgap differential between silicon andsilicon-germanium is maintained.

SUMMARY OF THE INVENTION

The above and other objects and advantages of the present invention areachieved by a heterojunction bipolar transistor having a thin, lightlydoped, silicon emitter disposed on a silicon-germanium base layer. Thelightly doped silicon emitter maintains the bandgap differential betweensilicon-germanium and silicon. The silicon emitter is fabricated suchthat it will be depleted at zero bias so that emitter-base capacitanceand emitter resistance is low. In one embodiment, a polysilicon contactlayer may be formed on the silicon emitter and can be heavily doped tofurther reduce emitter resistance. In a second embodiment, a thin ohmiccontact region may be formed in the silicon emitter instead of thepolysilicon contact layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an enlarged, cross-sectional portion of a firstembodiment of the present invention; and

FIG. 2 illustrates an enlarged, cross-sectional portion of a secondembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an enlarged, cross-sectional view of a portion of aheterojunction bipolar transistor in a first embodiment of the presentinvention. What is shown is a silicon layer 10 which acts as thecollector of the heterojunction bipolar transistor, a silicon-germaniumlayer 12 which acts as the base, a thin silicon layer 14 formed on thesilicon-germanium layer 12 which acts as the emitter, and a polysiliconlayer 16 which acts as the emitter contact. A metal contact layer 18 isformed on polysilicon layer 16.

Silicon collector layer 10 is preferably doped N-type, using arsenic orantimony as a dopant. Silicon-germanium base layer 12 is preferablydoped P-type, using boron as a dopant at approximately 1×10¹⁹ to 3×10¹⁹atoms/cm³. The thickness of silicon-germanium base layer 12 ispreferably approximately 500 angstroms. Thin silicon layer 14 ispreferably lightly doped N-type, using arsenic as a dopant atapproximately 8×10¹⁶ to 3×10¹⁷ atoms/cm³. The thickness of silicon layer14 is preferably approximately 500 to 1500 angstroms. The thickness anddoping of silicon layer 14 should be chosen such that silicon layer 14will be substantially depleted at zero bias during normal operation.This is necessary in order to reduce emitter-base capacitance andemitter resistance. If silicon layer 14 is too thick, for example 4,000angstroms as is described in the prior art, silicon layer 14 will not bedepleted at zero bias and emitter resistance will be high. Polysiliconlayer 16 is preferably heavily doped N-type using arsenic as a dopant.

After doping polysilicon layer 16, it is critical to anneal theheterojunction bipolar transistor by rapid thermal anneal to preventdegradation of the bandgap differential of silicon layer 14 andsilicon-germanium layer 12 by preventing the diffusion of arsenic andboron from silicon-germanium layer 12 and silicon layer 14. Polysiliconlayer 16 is heavily doped in order to further lower emitter resistance.In a preferred embodiment, polysilicon layer 16 is doped using arsenicas a dopant at a level of approximately greater than 1×10¹⁹ atoms/cm³.The above layers are formed using techniques well known in the art. Notethat only the active region of a heterojunction bipolar transistor isshown and described, however, this structure may be readily incorporatedinto many heterojunction bipolar process.

By including silicon layer 14 the metallurgical junction will be at theinterface between silicon layer 14 and silicon-germanium layer 12. Thus,the advantage of the bandgap differential is not lost as in the priorart. In addition, the heterojunction bipolar transistor of the presentinvention has low emitter resistance and low emitter-base capacitance.The low emitter resistance is obtained by forming a heavily dopedpolysilicon layer 16. The low emitter-base capacitance is obtained bydesigning silicon layer 14 to be substantially depleted duringoperation. The heterojunction bipolar transistor of the presentinvention is thus a very high speed device that can be used for highspeed digital and microwave applications.

FIG. 2 illustrates an enlarged, cross-sectional view of a portion of aheterojunction bipolar transistor in a second embodiment of the presentinvention. The same elements shown in FIG. 1 are referenced by the samenumerals. FIG. 2 illustrates a structure similar to that of FIG. 1,however, in FIG. 2 polysilicon layer 16 is not utilized. In addition,silicon layer 14 is preferably slightly thicker than in FIG. 1. Siliconlayer 14 is preferably approximately 1,000 to 2,500 angstroms so that ashallow emitter contact region 17 may be formed therein. Emitter contactregion 17 is preferably formed by using a heavy dose of arsenic, and isjust deep enough to provide ohmic contact to metal layer 18. In thissecond embodiment, as in the first embodiment, silicon emitter layer 14is thin enough so that it is substantially depleted during operation.Thus, the structure of FIG. 2 also exhibits good electrical properties.

As can be readily seen, the heterojunction bipolar transistor of thepresent invention maintains the bandgap differential, thus exhibits highemitter injection efficiency, reduced charge storage in the emitter,reduced or eliminated hole injection into the emitter. In addition theheterojunction bipolar transistor of the present invention exhibits lowemitter resistance and emitter-base capacitance by utilizing a thinsilicon emitter layer.

We claim:
 1. A heterojunction bipolar transistor, comprising:acollector; a silicon-germanium base disposed on the collector; a thinsilicon emitter disposed on the silicon-germanium base, wherein ametallurgical junction is maintained at the silicon-germanium base andthe thin silicon emitter interface; and a polysilicon layer disposed onthe thin silicon emitter.
 2. The heterojunction bipolar transistor ofclaim 1 wherein the silicon emitter is of a thickness where the thinsilicon emitter is substantially depleted at zero bias.
 3. Theheterojunction bipolar transistor of claim 2 wherein the silicon emitteris of a thickness of approximately 500 to 2,500 angstroms.
 4. Aheterojunction bipolar transistor comprising:a collector; asilicon-germanium base formed on the collector; a thin silicon emitterformed on the silicon-germanium base, wherein the silicon emitter has athickness and a doping level such that it is substantially depletedduring normal operation, and wherein a metallurgical junction ismaintained at the silicon-germanium base and the thin silicon emitterinterface; and a polysilicon layer formed on the silicon layer.
 5. Theheterojunction bipolar transistor of claim 4 wherein the silicon emitteris of a thickness of approximately 500 to 2,500 angstroms.
 6. Theheterojunction bipolar transistor of claim 4 wherein the polysiliconlayer is heavily doped.
 7. A heterojunction bipolar transistor,comprising:an N-type collector; a P-type silicon-germanium base formedon the collector; an N-type, thin, lightly doped silicon emitter formedon the silicon-germanium base, wherein a metallurgical junction ismaintained at the P-type silicon-germanium base and the N-type, thin,lightly doped silicon emitter interface; and an N-type, heavily dopedpolysilicon layer formed on the silicon layer.
 8. The heterojunctionbipolar transistor of claim 7 wherein the thin, lightly doped siliconemitter is substantially depleted during normal operation.
 9. Theheterojunction bipolar transistor of claim 8 wherein the thin, lightlydoped silicon emitter is doped at approximately 8×10¹⁶ to 3×10¹⁷atoms/cm³.
 10. The heterojunction bipolar transistor of claim 7 whereinthe heavily doped polysilicon layer is doped at approximately 1×10¹⁹atoms/cm³.